The purpose of annealing glass is to remove internal stresses that might cause later breakage. Stresses are likely to be present because of unequal temperature distribution in the glass article while it is being made. Glass that has not been annealed may shatter from tension caused by uneven cooling. Annealing is done by reheating the glass and then gradually cooling it according to a planned time-and-temperature schedule.
A somewhat related treatment, is the process of tempering glass. Tempering is a process in which a glass article that is already formed is reheated until almost soft. Then, under carefully controlled conditions, it is chilled suddenly by blasts of cold air, or alternatively by plunging it in oil or certain chemicals in a liquid state. This treatment makes the glass much stronger than ordinary glass. Glass which has been tempered may be up to five times as hard as ordinary glass.
Glass articles may be annealed or tempered by placing them on a metal belt which travels slowly through a heated enclosure called a lehr. Many lehrs measure four to eight feet wide inside and sixty to one hundred feet long. With previous equipment, thin articles could be annealed or tempered in thirty minutes or less.
Flat glass is used primarily for windows, but also finds uses in mirrors, room dividers, and many types of furniture. All flat glass is made in the form of flat sheets. However, some of it, such as that used in automobile windshields, is reheated and curved or sagged over molds. Flat glass can be classified as sheet glass, plate glass, and float glass.
Sheet glass is used in windows in most stores and offices. Plate glass and float glass are used where exceptionally clear and accurate vision is needed, as in automobile windshields and display windows. Sheet glass is taken from the melting furnace with a fire-polished surface and is given no additional treatment. Plate glass is sheet glass that has been carefully ground and polished to smooth the surface. Float glass is made by floating a ribbon of glass on the surface of molten metal instead of going through rollers and being ground and polished. This produces smoother, and more perfectly parallel surfaces. Both sides of the glass come out with a brilliant fire-polished finish.
Tempered safety glass is a single piece of glass that has been given a special heat treatment. It looks, feels, and weighs the same as ordinary glass. However, it can be up to five times as strong as ordinary glass. Tempered glass is used widely for glass doors in stores, for the side and rear windows of automobiles, and for other special purposes. It is hard to break even when hit with a hammer. When it does break, the whole piece of glass collapses into small, dull-edged, relatively harmless fragments.
Many different systems have been suggested in the past in order to accomplish the tempering of glass sheets and other items. Some of the glass tempering systems which have been suggested previously are discussed hereinbelow.
McMaster et al U.S. Pat. No. 3,015,910 discloses a toroidal furnace with a rotary conveyor extending through the inner wall of the furnace. The conveyor moves intermittently, while fans circulate heated air to the bottom of glass supported on a frame. The blast head has box-like jaws which close when heated glass is in position. The glass is subjected to oscillating air jets connected to a suction fan.
Nitschke et al U.S. Pat. No. 3,087,316 discloses an interrupted toroidal furnace, with a Geneva-type mechanism to index a rotary conveyor, open and close doors of individual furnace sections, and pull a blast head radially inward. The blast head oscillates when in position, and is fitted with air jets perpendicular to a curved glass surface, arranged in staggered rows of diagonally-oriented parallelograms elongated in the direction of oscillatory movement. Molds for bending the glass as it softens in the furnace have weighted members which press against the under surface of the glass.
Nitschke et al U.S. Pat. No. 3,130,032 discloses an interrupted toroidal furnace, where glass to be treated hangs vertically from tongs. A series of sliding plates cover the opening in the top of the furnace. A continually-oscillating blast head is intermittently supplied with pressurized air.
McMaster et al U.S. Pat. No. 3,249,415 relates to the use of independent conveyor systems in a linear furnace, and an overhead conveyor to return a mold and finished part to a loading and unloading station. Bending of the glass is controlled by the gas pressure supplied to a burner adjacent a bend area in the glass. The conveyors are controlled by photocells sensing that the desired bending has occurred.
McMaster U.S. Pat. No. 3,253,899 discloses the use of heat-sensitive tension tapes to separate the mold from the glass.
McMaster U.S. Pat. No. 3,281,229 relates to the use of an independent support system for a shaped ceramic bed, perforated for the passages of gases to support, heat and form glass sheets.
McMaster et al U.S. Pat. Nos. 3,332,759, 3,338,697 and 3,399,042 relates to use of shutters to control circulation in a gas support bed furnace, the use of varying patterns of perforations in the bed, and a conveyor system which cooperates with a tilted bed to frictionally engage the lowermost edge of the glass.
McMaster U.S. Pat. No. 3,353,946 discloses a blast head structure where cooling air provides a centering force to a vertically-suspended glass sheet.
McMaster et al U.S. Pat. No. 3,423,198 discloses the use of particles of materials in tempering media to vary the thermal conductivity of the media.
McMaster et al U.S. Pat. Nos. 3,455,669, 3,455,699 and 3,455,671 disclose a blast head structure for supporting a sheet of glass on the cooling air.
McMaster U.S. Pat. No. 3,485,612 relates to an inclined heating support bed, and structure for oscillating a floating glass sheet by pushing it with fingers responsive to contact with the glass sheet.
Nitschke U.S. Pat. No. 3,485,616 discloses the use of sheet members interposed between frictional drive rollers and the edges of the glass to be treated.
McMaster U.S. Pat. No. 3,488,173 relates to an apparatus for oscillating a sheet of glass supported on a gas support bed. A tilted bed and vertical-axis drive rollers contact the lowermost edge of the glass sheet, where the rotation of the rollers is controlled by a feedback control system to produce constant accelerations and decelerations. Separate structure oscillates the rollers transverse to the axis of the bed.
U.S. Pat. Nos. 3,574,588, 3,700,425, 3,723,085 and 3,607,200 disclose a loop chain fitted with pusher bars extending into the furnace to push sheets of glass into a shuttling carrier. A shuttling frame removes the glass from the carrier, raises it against a mold, and conveys it to a blast head.
McMaster U.S. Pat. No. 3,607,187 relates to a vacuum mold which is lowered to pick up a sheet of heated glass, and shape it. The glass is then either moved to a blast head, or to a frame which moves it to a blast head, where it is removed from the frame and oscillated.
McMaster et al U.S. Pat. No. 3,806,312 discloses a conveyor system including a pair of metal belts which slide on two rows of flat ceramic blocks inside the furnace and rotate rollers disposed on top of the belts and trapped between projections from the ceramic blocks.
McMaster et al U.S. Pat. No. 3,947,242 relates to a horizontal cylindrical furnace fitted with the conveyor disclosed in McMaster et al U.S. Pat. No. 3,806,312. The upper portion of each furnace section is hung from counterbalanced chains, so that it can be moved from a lower, closed, operating position, to a upper, open, non-operating position by manual actuation of an associated handle.
McMaster U.S. Pat. No. 3,994,711 discloses a furnace with separate loading, heating, quenching and unloading conveyors. The load and unload conveyor rollers are lifted from drive chains to stop the conveyors. The furnace conveyor oscillates a distance at least twice the length of the glass load, and is constructed as shown in McMaster et al U.S. Pat. No. 3,806,312 but differing in the location and material of the sliding surface for the metal drive chains.
U.S. Pat. Nos. 1,856,658, 1,856,669 and 1,879,998 disclose glass treating systems which incorporate horizontal roller conveyors. These conveyors carry sheets of glass having discrete lengths horizontally through elongated furnaces which have a decreasing temperature gradient in a direction along which the glass is conveyed. The conveyor rollers are alternately rotated in a forward and rearward rotation so that the glass is conveyed along the decreasing temperature gradient in a "two steps forward and one step backward" manner to provide thereby appropriate heat treating of the glass. Such a furnace requires a large number of reversals of the direction of roller rotation in order to provide the proper treatment of the glass.
Bornor U.S. Pat. No. 3,447,788 relates to a furnace having a horizontal roller conveyor which reciprocates a workpiece to be heated a slight amount in order to avoid the concentration of heat on the same areas of the workpiece. The purpose of this reciprocation is to ensure uniform heating of the workpiece within the furnace and to prevent sagging when the workpiece being processed is in the fluid state.
Other previous suggestions are disclosed in Drake U.S. Pat. No. 2,140,282 and Littleton U.S. Pat. No. 2,326,044.
The present invention overcomes various deficiencies and problems with respect to the previous suggestions outlined above, and also with respect to commercially-available equipment.